Capacitive Fingerprint Sensor Circuit for Noise-Resistant Ridge Valley Detection
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Solution Overview
Problem
Traditional TFT fingerprint sensor pixel circuits face challenges in achieving accurate capacitance difference detection due to environmental noise and cross-talk, especially with only one transistor, which limits the precision of fingerprint recognition.
Innovation Solution
The capacitive fingerprint sensor employs a fingerprint capacitor, multiple transistors, an operational amplifier, a reference capacitor, and a multiplexer to differentiate between ridge and valley capacitances, utilizing a precharge phase and reset lines to enhance detection accuracy through a readout circuit and image processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If only one transistor is used in the pixel circuit, then the device complexity is reduced, but the measurement precision of capacitance difference deteriorates due to noise and cross-talk
Solution Approach 1:
The pixel circuit is segmented into multiple functional blocks: a first transistor for controlling the fingerprint capacitor, a second transistor for precharging, a third transistor for resetting the operational amplifier, and additional transistors for signal processing. This segmentation allows each transistor to perform a specific function, improving measurement precision while maintaining reasonable device complexity
Solution Approach 2:
An operational amplifier is introduced as an intermediary component between the fingerprint capacitor and the readout circuit. The operational amplifier amplifies the small capacitance difference signals, enabling accurate detection of ridge and valley capacitances while rejecting noise and cross-talk interference
2Reliability
If environmental noise and cross-talk are present, then the reliability of fingerprint detection deteriorates, but adding more transistors and circuit components increases device complexity
Solution Approach 1:
A precharging phase is implemented before the actual measurement, where a second transistor precharges the fingerprint capacitor and reference capacitor to a known voltage level. This preliminary action ensures that the capacitors start from a consistent state, reducing the impact of noise and cross-talk on measurement reliability
Solution Approach 2:
The circuit dynamically changes operating parameters including voltage levels, timing sequences, and transistor switching states during different phases (precharge, measurement, reset). These parameter changes optimize the signal-to-noise ratio at different stages, improving reliability without requiring excessive circuit complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration improves the accuracy of fingerprint detection by effectively distinguishing between ridge and valley capacitances, reducing the impact of noise and cross-talk, and enhancing the overall precision of fingerprint recognition.
Implementation Method 1
The capacitance of the ridge capacitor CFR is related to C1, and the equivalence capacitance of the valley capacitors CFV is related to C1//C2. The capacitances of C1 and C1//C2 are listed as follows: Normally, the capacitance of the ridge capacitor CFR is far greater than the capacitance of the valley capacitors CFV.
Data Source
AI summary
A capacitive fingerprint sensor comprises a fingerprint capacitor, a reference capacitor, a first transistor, a second transistor, a comparator and a multiplexer. The fingerprint capacitor has a capacitance that is either a valley capacitance CFV or a ridge capacitance CFR, wherein CFV is smaller than CFR. One end of the reference capacitor CS is connected to the fingerprint capacitor, and the other end is connected to a trigger signal, wherein the trigger signal is initiated only during a precharge phase. The first transistor is configured to control the fingerprint capacitor during a scan line period. The second transistor is configured to precharge the fingerprint and reference capacitors. One end of the comparator is connected to the second transistor. The multiplexer is connected to another input end of the comparator for providing a first voltage VA and a threshold voltage Vth.